This application is the national phase under 35 U.S.C. xc2xa7 371 of PCT International Application No. PCT/JP98/02489 which has an International filing date of Jun. 4, 1998, which designated the United States of America.
This invention relates to a process for producing rubber reinforced styrene-based resins. More particularly, it relates to a process for producing rubber reinforced styrene-based resins which makes it possible to produce, at a high speed, products with uniform qualities which are excellent in the balance among impact strength, appearance characteristics and coloring properties, without changing the applied mechanical shear force which controls the particle size.
To produce a rubber reinforced styrene-based resin being excellent in coloring properties, impact strength and appearance characteristics, it is required to regulate the particle size and particle size distribution of rubber particles dispersed in the resin and the molecular weight of the continuous phase each within an appropriate range. It is a well known fact that the particle size of rubber particles is a particularly important predominant factor. The appearance of a resin is improved with a decrease in the rubber particle size. However, the impact strength of the resin is decreased with a decrease in the rubber particle size. It is generally considered that optical rubber particle size is from 0.8 to 3 xcexcm.
In general, the rubber particle size is determined at a certain point with a conversion ratio of styrene-based monomer of from 5 to 20% under a circumstance where a styrene solution of a rubbery polymer becomes a dispersion phase while a styrene solution of a polystyrene resin becomes a continuous phase (i.e., phase transformation occurs) by a shear of mechanical stirring and these are further subjected to a mechanical shear. This phase transformation is not completed instantly but proceeds over a considerably long time and large space. Therefore, the shear force for achieving the desired rubber particle size is determined depending on the rotary speed of and retention time in the reactor, mixer, etc. by which the shear force is applied.
To produce products of the same quality at a high speed by using the same polymerization apparatus, it is, therefore, needed to increase the amount of a stock solution fed into a reactor. In this case, however, the retention time in the reactor is shortened and, in its turn, the shear force suffered at the phase transformation is lowered, which results in an enlarged particle size. To overcome this problem, it has been a general practice to elevate the rotary speed of a stirrer by which the rubber particle size is regulated in order to enhance the mechanical shear force, whereby the resulting rubber particle size is adjusted to the desired value. To largely elevate the production speed, however, a large shear force should be applied. Therefore, it is needed to preliminarily enhance the capacity of the apparatus (e.g., mechanical strength, motor ability, etc.), which may be highly difficult due to the design of the apparatus used. So long as a certain level of mechanical strength can be ensured, the shear force can be enhanced by elevating the rotary speed. Over a critical point, however, the flow of a polymer liquid per se in the rotational direction becomes increased. As a result, the shear effect is saturated, and hence the desired rubber particle size may be hardly achieved merely by the mechanical shear force.
In order to produce products of the same quality at a high speed with the use of the same polymerization apparatus, there has been another method, besides the mechanical shear force, which is constituted by lowering the molecular weight of a rubbery polymer used. However, it is not desirable to lower the molecular weight of the rubbery polymer, since the impact strength, etc. of the obtained rubber reinforced styrene-based resin might be deteriorated thereby.
On the other hand, a process of the present invention is similar to the production process disclosed in JP-A-63-113009 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d). The process disclosed in this publication is directed to an object of improving the mechanical strength (e.g., impact strength) of a resin by increasing the content of a styrene-based polymer contained in dispersed rubber particles. Therefore, it is stated therein that the amount of the polymer returned from a circulating reactor into a mixer is from 12 to 35 parts by weight, preferably from 17 to 30 parts by weight, per 100 parts by weight of the polymer fed from the prepolymerizer into the mixer. When the amount of the circulated liquid falls within this range, the rubber particle size in the product cannot be changed and thus the productivity cannot be elevated owing to the circulation ratio. Accordingly, the present invention completely differs from JP-A-63-113009.
On the other hand, JP-B-59-17725 (the term xe2x80x9cJP-Bxe2x80x9d as used herein means an xe2x80x9cexamined Japanese patent publicationxe2x80x9d) discloses a production process which involves no such prepolymerization step as employed in the present invention. By this process, it is impossible to achieve the objects of the present invention in terms of the balance among impact strength, appearance characteristics and coloring properties.
An object of the present invention is to provide a process for producing a rubber reinforced styrene-based resin which makes it possible to produce, at a high speed, products excellent in the balance among impact strength, appearance characteristics and coloring properties, without changing the mechanical shear force which controls the particle size.
The present inventors have conducted extensive studies and consequently found out that the above-mentioned object of the present invention can be achieved by adjusting polymer concentrations in each of a prepolymer liquid and a circulated liquid and the mixing ratio of these two liquids to an appropriate range.
Accordingly, the present invention provides a process for producing a rubber reinforced styrene-based resin which comprises the following steps:
a step of prepolymerizing a styrene-based monomer stock solution containing from 2 to 15% by weight of a rubbery polymer;
a step of feeding the prepolymer liquid prepolymerized in the above step into to a circulating reactor via mixing means;
a step of circulating the polymer liquid from the circulating reactor back into the mixing means at a flow amount of from 0.5 to 3 times the flow amount of the prepolymer liquid fed into the mixing means, to substantially uniformly mix the prepolymer liquid and the polymer liquid by the mixing means; and
a step of further polymerizing the polymer liquid polymerized in the circulating reactor, in a plug-flow type reactor.
As an embodiment of the present invention, examples can be exemplified as follows:
a production process, using a polymerization apparatus comprising: a prepolymerizer; mixer and a circulating reactor connected with a circulation line; and a plug-flow type reactor, which process comprises: continuously feeding a stock solution containing from 2 to 15% by weight of a rubbery polymer dissolved therein into the prepolymerizer; feeding the prepolymerized liquid into the mixer; in this mixer, mixing the prepolymer liquid with circulated liquid that is circulated from the circulating reactor at a flow amount of from 0.5 to 3 times (circulation flow ratio: volume ratio) the flow amount of the fed prepolymer liquid; particulating and uniformly dispersing the rubbery polymer in the mixed liquid by mechanical shear force; and then feeding the polymer liquid into the plug-flow type reactor, to thereby effect a polymerization;
a process for producing a rubber reinforced styrene-based resin as described above, wherein the polymer concentration in the circulated liquid fed from the circulating reactor into the mixing means is adjusted to from 3.7A to 45% by weight, provided that A represents the concentration (expressed in % by weight) of the rubbery polymer in the stock solution; and
a process for producing a rubber reinforced styrene-based resin as described above, wherein the polymer concentration fed from the prepolymerizer into the mixing means is adjusted to from 1.2A to 2.8A % by weight, provided that A represents the concentration (expressed in % by weight) of the rubbery polymer in the stock solution.